Investment in renewable energy is rapidly increasing worldwide. This is in response to a number of global challenges and concerns, including climate change, increasing energy demand and energy security. The investment is widely spread over the leading renewable energy technology sectors such as wind, solar, biofuels, biomass and fuel cells. Among those, biofuels and biomass use the same principle as that used in traditional electric utilities for electric power generation, but wind, solar, and fuel cells employ completely different technologies and usually require power electronic converters for grid integration of those renewable resources.
There are basically two converter structures used for grid integration of renewable energies: a DC/DC/AC converter for, for example, solar and fuel cell applications; and, an AC/DC/AC converter for, for example, wind power applications. FIGS. 1A and 1B show different embodiments of an exemplary AC/DC/AC converter structure in wind power applications. For both, the converter is built by two self-commutated back-to-back pulse-width modulation (PWM) converters, a converter on the renewable energy source side and a converter on the grid side, with an intermediate DC voltage link. By controlling the converters on both sides, it is expected to adjust renewable energy source characteristics so as to achieve maximum power conversion capability and to control its power generation with less fluctuation. However, to meet this need, the grid-side or front-end converter is controlled in such a way as to maintain a constant DC-link capacitor voltage and to keep the converter operation with a desired power factor while the unbalance and harmonics in the grid system must be low. For example, Codd as well as Xu and Wang (I. Codd, “Windfarm Power Quality Monitoring and Output Comparison with EN50160”, Proc. of the 4th Intern. Workshop on Large-scale Integration of Wind Power and Transmission Networks for Offshore Wind Farm, 20-21 Oct. 2003, Sweden; and L. Xu, and Y. Wang, “Dynamic Modeling and Control of DFIG Based Wind Turbines under Unbalanced Network Conditions”, IEEE Transactions on Power Systems, Vol. 22, No. 1, February 2007; both fully incorporated herein by reference), have shown that wind farms periodically experience unbalance and high harmonic distortions, which not only presents challenges to the proper operation of electric power grid but also results in a large number of generator trips.
Current grid-side PWM converter control is generally a nested-loop controller operating in a grid AC voltage reference frame as shown by FIG. 2 using a current-regulated voltage-source PWM converter scheme. In this embodiment, the d-q voltage control signals in the grid-side controller are obtained by comparing the d- and q-current setpoints to the actual d- and q-currents to the grid as shown in the second stage controller in FIG. 2, and are final control actions actually applied to the grid-side converter. Present technology uses d-axis voltage, νd1* for DC link voltage control, and q-axis voltage, νq1*, for reactive power control. The control of the grid-side converter is important because if the control goals of the grid-side converter cannot be met, all other control objectives will be affected in a renewable energy system.
The existing technology for grid-side PWM converter control has theoretical deficiencies in nature. It can cause major periodic unbalance to influence the proper operation of both renewable energy and grid systems. Therefore, control systems and methods are desired that overcome challenges present in the art, some of which are described above.